Hi,
I had this happen to me with my first child. I was in labour for two days and then they finally decided to do a csection. During the section they had a hard time removing my placenta because it was stuck/grown into my uterus. I bled quite alot but my blood level was hight to begin with so i didn't need a blood transfusion. They thought they had removed it all but i ended up back in the hospital a week later with a heavy bleed. They gave me a shot that would help all the remainder of the placenta die off. It did and passed it about a week later with no futher difficulties. I'm not pregnant with my second child and terrified it will happen again if not worse. I had no risk factors so it totally took my drs by surprise. My pregnancy itself was fine though, no problems.
I have read about this on the net to and i believe that you can still give birth but there are a few risks. You should discuss this with your doctpr or midwife so that you can share with them your concerns.